Maleic anhydride is of significant commercial interest throughout the world and is extensively used in the manufacture of alkyd resins and as an intermediate for chemical synthesis among other things. Fumaric acid is also of significant commercial interest and is used in the manufacture of inks, in paper sizing, in unsaturated polyesters, and as a food additive among other things.
Maleic anhydride is produced industrially by gas phase oxidation of four-carbon hydrocarbons such as benzene, butane, butenes, and butadiene, etc., in the presence of suitable catalysts and oxygen. The maleic anhydride thus formed may be separated subsequently in the solid or liquid state in suitable separators. Part of the maleic anhydride formed passes in gas form through a separator and must be washed out of the gas stream with water or suitable absorption agents. When water is used as a washing agent, concentrated aqueous maleic acid solutions result. This maleic acid must then be converted to maleic anhydride.
A common method used to convert the maleic acid to maleic anhydride is to remove the water by azeotropic distillation under atmospheric pressure in the presence of a suitable organic solvent entrainer, such as o-xylene or another xylene, pseudocumene, etc. which removes the solution water and splits off one mole of water to convert the maleic acid to maleic anhydride. The organic solvent may then be removed by distillation and the crude maleic anhydride further purified if desired.
Another method of converting the maleic acid to maleic anhydride is to remove the water by vacuum distillation during which the maleic acid is converted to maleic anhydride at temperatures of 150.degree. C. to 160.degree. C. (302.degree. F. to 320.degree. F.). In this method the maleic acid to maleic anhydride conversion takes place by thermal treatment in the presence of a solvent which may be o-xylene or another suitable organic solvent or maleic acid anhydride.
A major disadvantage of the thermal vacuum distillation is that the high temperatures needed for the thermal treatment cause a significant portion of the maleic acid to isomerize to fumaric acid. The fumaric acid, which is insoluble or only slightly soluble under reaction conditions, has a high melting point and causes clogging of valves, pipes and pumps, as well as deposits in the vessels, which represents a loss in yield and requires that the manufacturing installation be shut down periodically in order to clean the units and remove the fumaric acid that has collected and is fouling the connecting apparatus. Such shut downs are costly and reduce the efficiency of the maleic anhydride production process.
OIL GAS J., v.72, n. 36, 103 Sep. 9, 1974 discloses a continuous recovery of pure maleic anhydride wherein fumaric acid is removed down to a residual content of 0.1 wt. % by filtration in the first stage prior to dehydration. In the second stage the residual maleic acid in the crude mixture is converted to maleic anhydride and water, and the water and low boilers are removed. In a third stage the entrainer is removed by distillation, if necessary.
GB 2 098 612 discloses a process for production of fumaric acid from wash waters of exhaust gasses resulting from hydrocarbon oxidation which contain maleic acid. The process comprises a thermal pretreatment of the impure maleic acid solution, with optional concurrent concentration, with transformation of part of the impurities to facilitate their elimination from the final product, subsequent hot filtration, isomerization of the MA to FA using thiourea as catalyst for the isomerization, crystallization according to a pre-set time/temperature chart, removal of the mother liquors by filtration, subsequent dissolution in water or in a fumaric acid solution, decolorization, recrystallization, and drying.
U.S. Pat. No. 3,993,671 (Ramioulle) discloses a continuous process for the preparation of maleic anhydride from an aqueous solution of maleic acid formed in the production of maleic anhydride by the catalytic oxidation of aliphatic or aromatic hydrocarbons, with the continuous elimination of the impurities which accompany this maleic acid solution or which are subsequently formed in the conversion of maleic acid into maleic anhydride comprising the stages:
a. continuously heating the starting aqueous solution of maleic acid in a concentration zone kept at a temperature of 100.degree. to 150.degree. C. and at a pressure of 400 to 760 mm. Hg. in order to obtain molten maleic acid containing 0 to 10% by weight of water and water vapor, and washing said water vapor in order to recover entrained maleic acid therefrom, the aqueous solution of maleic acid thus formed being fed to state (c); PA1 b. continuously feeding the molten maleic acid obtained in stage (a) to a conversion zone kept at a temperature of 115.degree. to 165.degree. C. and at a pressure of 40 to 200 mm. Hg. and consisting of PA1 c. continuously or semi-continuously withdrawing from 0.5 to 5% by weight of the said reaction mixture present in said conversion zone of step (c) per hour, and feeding it to a dissolution and filtration zone, in which it is suspended in the aqueous solution of maleic acid coming from stages (a) and (d), filtering the suspension thus obtained in order to separate a solid cake of fumaric acid and resinous residues, which is discharged from the system, and recycling a liquid filtrate consisting of an aqueous solution of maleic acid, to the starting aqueous solution of maleic acid; PA1 d. continuously condensing the gaseous phase produced in stage (b) at a temperature above the dew point of the water vapor present, the condensate thus obtained being maleic anhydride with a purity of at least 99%, which is recovered as the product of the process, while washing with water the residual water vapor, which still contains entrained maleic anhydride, in order to form an aqueous solution of maleic acid, which is recycled to state (c). PA1 (a) deriving a composition comprising maleic anhydride and fumaric acid, from a system for the catalytic oxidation of butane or another hydrocarbon after dehydration; PA1 (b) cooling said composition sufficiently to precipitate the fumaric acid; PA1 (c) separating the fumaric acid precipitate from step (b) from the maleic anhydride, and recovering the fumaric acid; and PA1 (d) recovering the maleic anhydride. PA1 (a) deriving a composition comprising maleic anhydride and fumaric acid, from a system for the catalytic oxidation of butane or another hydrocarbon after dehydration; PA1 (b) cooling said composition sufficiently to precipitate the fumaric acid; PA1 (c) separating the fumaric acid precipitate from step (b) from the maleic anhydride, and recovering the fumaric acid; PA1 (d) recovering the maleic anhydride and, optionally, distilling said maleic anhydride to obtain pure maleic anhydride and recovering the pure maleic anhydride; and PA1 (e) optionally, removing residual maleic anhydride from the recovered fumaric acid to obtain purified fumaric acid and then recovering the residual maleic anhydride. PA1 (a) deriving a composition comprising molten maleic anhydride fumaric acid, and an entrainer, wherein a portion of the fumaric acid is dissolved in the maleic anhydride, from a system for the catalytic oxidation of butane or another hydrocarbon after dehydration; PA1 (b) cooling said composition sufficiently to precipitate the fumaric acid; PA1 (c) separating the fumaric acid precipitate from step (b) from the maleic anhydride/entrainer composition and, optionally, stripping the fumaric acid with a stripping gas to remove residual maleic anhydride, before recovering the fumaric acid; PA1 (d) removing the entrainer from the from step (c) to produce a crude maleic anhydride product substantially free from said entrainer and, optionally, recovering the entrainer, and PA1 (e) distilling said crude maleic anhydride to obtain pure maleic anhydride and recovering the maleic anhydride. PA1 (a) deriving a composition comprising molten maleic anhydride fumaric acid, and a xylene, wherein a portion of the fumaric acid is dissolved in the maleic anhydride, from a system for the catalytic oxidation of butane or another hydrocarbon after dehydration; PA1 (b) cooling said composition sufficiently to precipitate the fumaric acid; PA1 (c) separating the fumaric acid precipitate from step (b) from the maleic anhydride/entrainer composition by filtration and, optionally, stripping the fumaric acid with nitrogen to remove residual maleic anhydride before recovering the fumaric acid; PA1 (d) removing the xylene from the maleic anhydride/entrainer composition from step (c) to produce a crude maleic anhydride product substantially free from said xylene and, optionally, recovering the xylene, and PA1 (e) optionally, distilling said crude maleic anhydride to obtain pure maleic anhydride and recovering the maleic anhydride.
I. a liquid suspension composed of a reaction mixture containing about 1 to 20% by weight of maleic acid, 0 to 30% by weight fumaric acid, 99 to 55% by weight of maleic anhydride, and 0 to 5% by weight of resinous residues, and; PA2 ii. a purified gaseous phase containing maleic anhydride and water vapor, the amount of molten maleic acid added in an hour to the said reaction mixture representing from 10 to 50% of the weight of said reaction mixture;
This process differs from the process of the present invention. It uses a two-step dehydration process with a thin film evaporator followed by a still-dehydrator which is operated under vacuum. The MAN product along with water leaves the dehydrator in a gaseous stream overhead. A bottoms purge from this step, containing maleic acid and fumaric acid goes to the filter. The stream being filtered is different; it filters a maleic acid solution which is then recycled to the process. In Ramioulle's process, a purge of the reaction mixture in the still-dehydrator is combined with aqueous maleic acid solutions from other points in the process. This aqueous mixture of maleic acid, fumaric acid and other residue is then passed through the filter. The filtrate is maleic acid and water which is returned to the process upstream.
U.S. Pat. No. 3,657,333 discloses a process for the production of fumaric acid from the residue formed in equipment for refining maleic acid or maleic anhydride especially from the wash waters used in the cleaning of equipment for refining maleic acid or maleic anhydride. The process comprises filtering a hot aqueous solution of the raw fumaric acid (e.g. wash water from the aforementioned equipment) optionally after isomerization of its maleic acid content to fumaric acid, separating from the filtrate the prepurified fumaric acid which crystallizes out upon cooling, and drying the fumaric acid from the filtrate and subjecting it to a thermal treatment at 170.degree./C. to 240.degree. C. in an oxygen poor atmosphere, dissolving the resulting product in hot water, filtering the hot solution and treating the filtrate with decolorizing agents.
U.S. Pat. No. 4,191,695 discloses a process for obtaining maleic anhydride in which the formation of fumaric acid by-product is avoided by reacting the maleic acid precursor with an organic carboxylic acid anhydride so that an H.sub.2 O exchange is carried out between the maleic acid and the organic carboxylic acid anhydride giving as end products maleic anhydride and organic acid. The reaction is said to be conducted at such low temperatures that the customary impurities formed at higher temperatures no longer result and the isomerization of maleic acid into fumaric acid does not occur.
The process of the present invention avoids the problems of equipment fouling caused by FA and provides a process for coproducing FA along with MAN. In the process of the present invention, FA is recovered as a co-product with MAN, when MAN recovery is an aqueous recovery. FA is recovered by employing a separation operation to remove the FA from a stream of molten MAN. The FA is recovered as a solid. If desired, residual MAN may then be removed from the FA solids, by stripping, washing, or other means, to provide a high purity FA. In the preferred process, a sintered metal filter is used. This type of filter withstands the high temperature and corrosive environment when the correct metallurgy is used. The process involves allowing FA to form in a holding tank, pumping the stream containing FA through the filter to recover the FA, recycling the FA-free stream in part to the holding tank with the rest going to further processing. In the preferred form of the process, at least two filters are used in order to provide continuous process performance. When the first filter is full of FA, the flow is switched to the other filter; the FA solids which have been collected on the first filter are stripped with inert gas to remove residual MAN, cooled in a conveying device, and packaged in a shipping bag. When the second filter is full of FA, the flow is switched back to the first filter and the FA solids on the second filter are stripped with gas, cooled in a conveying device, and are ready for packaging. Optionally the FA may be purified further by washing with water following stripping off of the residual MAN with gas.